Re: performance of virtual functions compared to virtio

[David Ahern <dsahern@gmail.com>]

On 04/21/11 02:07, Avi Kivity wrote:
> On 04/21/2011 05:35 AM, Alex Williamson wrote:
>> Device assignment via a VF provides the lowest latency and most
>> bandwidth for *getting data off the host system*, though virtio/vhost is
>> getting better.  If all you care about is VM-VM on the same host or
>> VM-host, then virtio is only limited by memory bandwidth/latency and
>> host processor cycles.  Your processor has 25GB/s of memory bandwidth.
>> On the other hand, the VF has to send data all the way out to the wire
>> and all the way back up through the NIC to get to the other VM/host.
>> You're using a 1Gb/s NIC.  Your results actually seem to indicate you're
>> getting better than wire rate, so maybe you're only passing through an
>> internal switch on the NIC, in any case, VFs are not optimal for
>> communication within the same physical system.  They are optimal for off
>> host communication.  Thanks,
>>
> 
> Note I think in both cases we can make significant improvements:
> - for VFs, steer device interrupts to the cpus which run the vcpus that
> will receive the interrupts eventually (ISTR some work about this, but
> not sure)

I don't understand your point here. I thought interrupts for the VF were
only delivered to the guest, not the host.

David

> - for virtio, use a DMA engine to copy data (I think there exists code
> in upstream which does this, but has this been enabled/tuned?)
>